A known valve control of this type allows to detect the position of valve disks, e.g. at an accuracy of 0.1 mm (Journal dei 1-2/2001, page 14). Such accuracy presupposes a sensor system which makes possible four different positions of detection.
U.S. Pat. No. 5,704,586 describes a device for the control of a valve in which a rectilinearly operating position indicator detects the respective position of the valve rod and transmits it to a data registration device. The latter is in communication with a control unit which, in turn, has an access to a solenoid valve which engages a supply line for compressed air to the driving cylinder of the valve. The valve rod is connected to the closing member of the associated valve so that the position of the valve can be detected. This known device monitors the valve operation between a number of fixed valve positions. To this end, means are provided to store these desired valve positions in a data memory. In addition, the device has checking devices to compare the current position indications of the position indicator to the fixed switching positions of the valve which are deposited in said data memory. Furthermore, the control has programs in which the alterations of the position indications are deposited for the fixed valve positions during the repeated valve switching operations with the programs balancing out those alterations which are within a tolerance range of the deposited position indicator. Such an assembly has the advantage that the position indicator can adjust itself within the predetermined tolerance range so that influences exerted on the stroke of the valve can be detected and compensated (a so-called self-adjustment), for example, because of a change of temperature, wear to the sealing or change to the direction of afflux to the closing member, which can lead to an altered closing position of the valve disk, for example.
In the conventional technique, for example, the closed and opened positions of the valve are detected by an associated stationary position detector each. Now, for example, when the closed position of the valve disk changes and, hence, so does the information associated with the original closed position because of the aforementioned influences the stationarily arranged position detector cannot identify such an alteration. For example, when the position detector is a so-called microswitch which corresponds with an annular groove within the valve rod the groove releasing this microswitch will give a release signal in the original closed position although the valve is not completely closed yet because of wear to the sealing.
The fundamental solution embodied in U.S. Pat. No. 5,704,586 for a valve control, in the course of the so-called self-adjustment at certain time intervals, now allows to move the closing member and, thus, the valve rod connected thereto into the closed position which is actually possible and has perhaps been altered by the aforementioned influencing factors, and the system for the position detection is readjusted, if necessary, to the altered closing conditions which now exist.
A sensor system which functions with a non-contactingly working position indicator, on one hand, and can adapt itself to varying operating conditions of the valve within predefined tolerance ranges, on the other, is referred to as a so-called “floating measuring system” in relevant technical journals (confer Journal CIT/plus/4/01, pages 42 to 46; Journal dei 1-2/2001, page 14; Journal Brauwelt No. 46/47 (2000), pages 2018 to 2023).
The valve control described in the aforementioned reference documents makes possible four different detection positions of the valve (fixed valve positions) which are mentioned there. Apart from the two detection positions for the closed and opened positions of the valve, it also allows to identify those positions in which the valve seat is opened by a gap via the one or other valve disk for the purpose of the so-called seat cleaning if the valve to be controlled is a so-called double-seated valve which has two closing members movable relative to each other, i.e. a valve disk which is independently driven and a so-called double disk that is desmodromically driven by it and, hence, is dependently driven.
The realization of a so-called floating measuring system presupposes that there is a position indicator which works in a reliably non-contacting, preferably rectilinear way. A device of this type is known from WO 96/10731, for example. The device which is referred to as a position detection device there serves for determining a position of an element, this element being a valve tappet having a longitudinal axis, by analogy with a valve control of the previously mentioned type, and the tappet being movable along its longitudinal axis. As the cited document sets forth a magnet is mounted on the element. Moreover, an array of several magnetic-field converters is provided which are disposed in a rectilinear relationship in parallel with and in the vicinity of the path length defined by the valve tappet with each of these field converters, which preferably are magnetoresistive, predetermining a bipolar output signal when the magnet approaches the field converter, runs past it, and moves away from each one. The array of field converters predetermines a multiplicity of output signal values for a position of the magnet which are resorted to for a determination of this position and, thus, the valve rod to which the valve tappet is attached. The magnetic-field converters concerned, for example, are Hall sensors which when the magnetic field changes produce a voltage variation proportional to such change. The position detection device described in the document WO 96/10731 makes it possible to generate an electric output signal proportional to the valve tappet movement which can be evaluated in a valve control for position identification in accordance with U.S. Pat. No. 5,704,586.
In the device for the control of the valve operation between a number of fixed valve positions according to U.S. Pat. No. 5,704,586, the current position messages of the position detector are compared to the fixed position messages which are deposited in the data memory. Any changes that could possibly occur here are calculated and are compensated as far as they are within a predetermined tolerance range around a deposited fixed position message. This means that the field of tolerance which has been predetermined will be shifted to a predefined, limited extent in response to the change found to exist. In conjunction with this compensation, it cannot be made out distinctly whether the alterations found to exist were provoked by real shifts of the fixed valve setting (end positions, intermediate positions) which have occurred in the meantime or whether those completely or partially are changes in the electronic measuring system. Therefore, it is absolutely possible that although changes are determined for the actual fixed valve positions those changes are not initially brought about by the closing member position with regard to the associated seating surface. Accordingly, it is imaginable that a compensation is made even if there are no sufficient relevant reasons justified by the real conditions on the valve.